The present invention relates generally to transistors, and, more particularly, to a heterojunction bipolar transistor (HBT) having an improved emitter-base junction.
Heterojunction bipolar transistors (HBTs) have become state of the art, particularly in npn form, for applications in which high switching speeds and high frequency operation are desired. The emitter in an HBT has a bandgap wider than the bandgap of the base, thus creating an energy barrier in the valence band at the emitter-base junction that inhibits the unwanted flow of holes from the base region to the emitter region. This arrangement increases the emitter injection efficiency, current gain and operating frequency of the HBT.
First generation commercial HBTs were based on a gallium-arsenide (GaAs) substrate and semiconductor materials lattice matched to GaAs. Next generation HBTs are likely to be based on an indium-phosphide (InP) substrate and semiconductor materials lattice matched to InP. Typically, the base of such an HBT is fabricated from either the indium-gallium-arsenide (InGaAs) material system or the gallium-arsenide-antimonide (GaAsSb) material system, with the collector and the emitter fabricated from, for example, InP, aluminum-indium-arsenide (AlInAs) or InGaAs. HBTs that are fabricated using GaAsSb as the base material and AlInAs as the emitter material offer certain advantages over HBTs in which the base material is GaAsSb and the emitter is material is InP. For example, the conduction band energy line-up between AlInAs and GaAsSb (emitter-base) provides certain advantages, such as higher current-density operation, and hence higher frequency operation. Unfortunately, it is difficult to grow a good AlInAs/GaAsSb interface.
FIG. 1 is a graphical illustration showing an energy band diagram 11 of a conventional InP emitter/GaAsSb base/InP collector HBT under modest forward electrical bias on the emitter-base junction. The vertical axis 12 represents the energy level and the horizontal axis 14 represents distance. That is, the thickness of the material that respectively comprises the emitter region 22, the base region 24 and the collector region 26. A heterojunction bipolar transistor (HBT) with a GaAsSb base and InP collector has a type-II band lineup at the collector-base junction 32 as shown. The energy discontinuity xcex94Ec in the conduction band 16 is about 0.18 electron Volts (eV) and the energy discontinuity xcex94Ev in the valence band 18 is about 0.76 eV. This is an essentially ideal band lineup for this junction for the following reasons. A small ballistic energy xcex94Ec is imparted to collected electrons and there is a large valence-band discontinuity xcex94Ev at the metallurgical base (base-collector junction 32) that minimizes hole injection into the collector region 26 even at low or positive collector bias. Since the wide-bandgap InP extends throughout the collector region 26, avalanche breakdown is minimized.
Other variations of HBTs lattice-matched to InP, but with a base layer different from GaAsSb fail to offer these advantages. For example, the use of the same structure but with an InGaAs base has the large valence-band discontinuity xcex94Ev at the metallurgical base and the benefit of the wide-bandgap InP, but presents a barrier to electron collection, which could result in undesirable stored charge in the base. This compromises the frequency response and maximum current of the device. Any scheme to eliminate this barrier compromises the desired features of the large valence-band discontinuity xcex94Ev at the metallurgical collector-base junction, and the benefit of the wide-bandgap InP.
Furthermore, in HBTs having a GaAsSb base and InP emitter (as shown in FIG. 1) the type II band lineup leads to two undesirable features. Both are related to the discontinuity in the electron concentration across the heterojunction of exp(xe2x88x92qxcex94Ec/kT), where q is the electron charge, xcex94Ec is the conduction band discontinuity, k is Boltzmann""s constant, and T is the absolute junction temperature. Since xcex94EC is approximately 0.18xc2x10.1 eV, the ratio of electron concentration across the discontinuity is in the range of 2xc3x9710xe2x88x925 to 5xc3x9710xe2x88x922 at room temperature.
The first undesirable feature is lowered current gain. Below some limiting injection level, it can be shown that interface recombination at the metallurgical junction (the emitter-base junction 28) depends on the electron concentration on the emitter side of the junction and on the interface trap properties.
The interface current density jinterface=qnemittervinterface, where nemitter is the electron density on the emitter side of the interface and where vinterface is the interface recombination velocity. The interface recombination velocity vinterface="sgr"nvthermalNtraps+Ksxe2x80x94ixe2x80x94radpbase, where "sgr"n is the cross-section for capture of an electron by an interface trap, vthermal is the thermal velocity of electrons, Ntraps is the trap concentration as a density per unit area, Ksxe2x80x94ixe2x80x94rad is a constant that describes the proportionality of spatially indirect radiative recombination at the interface, and pbase is the hole concentration on the base side of the interface. The total interface recombination velocity is thus due to recombination through traps, and through spatially indirect radiative recombination. The material interface, as it can be practically grown, will not be electrically perfect. For example, there may be impurities or imperfections at the interface that lead to spatially localized states inside the energy gap. Electrons or holes that land in these spatially localized states cannot move around (unlike electrons or holes in the conduction or valence bands), and these spatially localized states have a potential energy between that of the valence and conduction bands. These spatially localized states can alternately trap electrons and holes, thereby providing a path for recombination. This is conceptually similar to Schockley-Read-Hall recombination. Spatially indirect recombination is band-to-band recombination between electrons that are localized on one side of a type-II heterojunction (in this example the InP side) and holes that are localized on the other side (in this example the GaAsSb side). The recombination is referred to as spatially indirect because the electrons and holes are separated according to classical physics. According to quantum physics the electrons and holes are not perfectly localized. They are represented by wave functions that slightly overlap. Therefore, some recombination occurs. Both of these effects are known to those having ordinary skill in the art.
The injection current density jinjection=qnbasevbase, where nbase is the injected electron concentration on the base side of the emitter-base junction and where vbase is the electron velocity through the base. The ratio jinjection/jinterface=vbasenbase/vinterfacenemitter represents an upper limit to the current gain of the transistor. The ratio of electron density on either side of the metallurgical junction leads to an effective multiplication of the interface recombination velocity by exp (qxcex94Ec/kT), directly affecting the current gain.
The second undesirable feature in an HBT having a GaAsSb base and InP emitter is a reduction of the current at which current gain compression occurs. In typical HBT""s, a relatively low emitter doping Ne is used to reduce the emitter-base capacitance. For example, the use of a 4-8xc3x971017 cmxe2x88x923 emitter doping places a hard upper limit, of Ne exp (xe2x88x92qxcex94Ec/kT), on the injected electron density in the base. This is illustrated by the energy band diagram 51 in FIG. 2, which represents the InP/GaAsSb/InP HBT of FIG. 1 at a strong forward bias on the emitter-base junction 68. As this bias is approached, the emitter capacitance becomes quite large and the frequency response is rapidly lowered. For purely diffusive transport, the electron velocity through the base, denoted by vbase, is on the order of 107 cm/sec in a typical microwave transistor. This leads to gain compression, in the presence of the electron discontinuity, at a current density in the range of 20 amperes/square centimeter (A/cm2) to 5xc3x97104 A/cm2. The experimental values are closer to the high end of this range, but still seriously limit the device operation by limiting the emitter charging frequency gm/(2xcfx80Ce), where gm is the dynamic emitter conductance and Ce is the emitter junction capacitance.
These problems can be reduced or eliminated by reducing or eliminating the conduction-band discontinuity by advantageously aligning the energy bands. However, a material system that actually results in such a band lineup, and that meets lattice-matching requirements, is difficult, or perhaps even impossible to produce without resorting to quaternary emitter or base materials. Quaternary materials, such as aluminum-indium-arsenide-phosphide (AlInAsP), are notoriously difficult to grow as high quality films. Lattice matching is problematic and the etch steps in the fabrication processes can also present difficulties. A different approach is to use an emitter material that has slightly higher conduction-band energy than GaAsSb. This approach will result in the formation of a conduction band spike at the emitter-base junction, which will increase the turn-on voltage. However, there will not be any current limiting, and the effect of the spike on the turn-on voltage can be reduced substantially by allowing the electrons to pass through the spike by tunneling, instead of requiring sufficient energy to overcome it. This is known as tunneling through the spike.
Therefore, there is a need in the industry for an HBT having an emitter material with a conduction band energy higher than the conduction band energy in the base, but that is easy to fabricate using existing technologies.
The invention provides an HBT having a base-emitter junction that exhibits the desirable properties of a GaAsSb/AlInAs interface, and that includes an intermediate layer in the emitter such that the intermediate layer contacts the GaAsSb base and the AlInAs emitter. The intermediate layer is sufficiently thin to be substantially electrically transparent, but sufficiently thick to provide a surface over which to grow the AlInAs emitter. The substantially electrically transparent intermediate layer may either be lattice matched to the GaAsSb base and the AlInAs emitter or be pseudomorphically grown so as to provide an apparent lattice-match to the GaAsSb base and the AlInAs emitter. The lattice in a pseudomorphic layer is stretched or compressed so that it matches the substrate lattice in the two dimensions perpendicular to the growth direction.